This invention is concerned with a video quality measure, in particular in situations where a video signal has been encoded using a compression algorithm.
When a video signal is to be transmitted from one location to another, it is known to encode or compress the signal using an encoding algorithm, such that the encoded signal can be transmitted using a lower bandwidth than would be needed without encoding. Upon reception, the encoded signal is decoded to retrieve the original signal. In many encoding techniques, a two dimensional cosines transform is performed, resulting in a series of transform coefficients, whose magnitude is quantized. So that the bandwidth can be allocated efficiently, the granularity of the quantisation, that is, the step size, is allowed to vary.
The process of encoding and decoding the video sequence can introduce distortion or otherwise reduce the quality of the signal. One way of measuring the level of distortion involves noting the opinion of viewers as to the level of perceptible distortion in a distorted video sequence, and averaging the results so as to obtain a Mean Opinion Score (MOS). However, this can be a time consuming process. As a result, it can be desirable to predict the loss of quality that a viewer will perceive in a video sequence. Although the degradation in the video quality as a result of encoding/decoding and the transmission process can be obtained by reference to the original sequence, such an approach is often inconvenient.
In predictive coding, the difference between the actual signal and the predicted one, known as the “prediction residual” may be transmitted. More usually, a quantised version of it is transmitted.
According to our co-pending international patent application WO2007/066066, there is provided a method of generating a measure of quality for a video signal representative of a plurality of frames, the video signal having: an original form; an encoded form in which the video signal has been encoded using a compression algorithm utilising a variable quantiser step size such that the encoded signal has a quantiser step size parameter associable therewith; and, a decoded form in which the encoded video signal has been at least in part reconverted to the original form, the method comprising the steps of: a) generating a first quality measure which is a function of said quantiser step size parameter; b) generating a second quality measure which is a function of the spatial complexity of at least part of the frames represented by the video signal in the decoded form; and, c) combining the first and second measures.
In predictive coding, the difference between the actual signal and the predicted one, known as the “prediction residual” may be transmitted. More usually, a quantised version of it is transmitted.
According to the present invention, there is provided a method of generating a measure of quality for a video signal representative of a plurality of frames, the video signal having: an original form; an encoded form in which the video signal has been encoded using a compression algorithm utilising a variable quantiser step size such that the encoded signal has a quantiser step size parameter associated therewith and utilising differential coding such that the encoded signal contains representations of the prediction residual of the signal; and a decoded form in which the encoded video signal has been at least in part reconverted to the original form, the method comprising:
a) generating a first quality measure which is dependant on said quantiser step size parameter according to a predetermined relationship;
b) generating a masking measure, the masking measure being dependant on the spatial complexity of at least part of the frames represented by the video signal in the decoded form according to a predetermined relationship; and
c) generating a combined measure, the combined measure being dependant upon both the first measure and the masking measure according to a predetermined relationship;
wherein the method also includes
generating a second measure which is dependant on the prediction residual of the signal according to a predetermined relationship;
identifying one or more regions of the picture for which the second measure exceeds a threshold;
and wherein the masking measure is dependant on the spatial complexity of the identified region(s) according to a predetermined relationship.
Other aspects of the invention are set out in the claims.